A photolithography technique is known as a technique according to which a material formed on a substrate is etched using a photoresist pattern obtained by photomask-utilized light-exposure and development of a photoresist, as a mask. In a quest for improved resolution, a thinner photoresist becomes necessary, which has led to a situation where the thinner photoresist alone cannot offer sufficient etching resistance. To deal with this problem, a technique has come into wide use, according to which a photoresist pattern is transferred to a mask material, such as silicon nitride film that can be processed by the thinner photoresist, and then a work piece formed on a substrate is etched, using this silicon nitride film as a mask, to form a pattern. Such a silicon nitride film is referred to as hard mask.
A demand for microfabrication and high densification of semiconductor memories, etc., has been so intensified in recent years that the pace of an improvement in resolution through development of lithographic techniques including exposure systems and photoresist materials is not fast enough to meet such a demand. Under these circumstances, techniques by which patterns are formed at a pitch smaller than a lithography resolution limit, utilizing a hard mask, has become widely noticed.
According to one of such techniques, a core pattern is formed first, and a spacer is formed on the side walls of the core pattern, and then a hard mask material is buried in recessions between different parts of the spacer to form a gap pattern. Subsequently, the spacer is eliminated selectively to form a pattern between the core pattern and the gap pattern, the formed pattern being identical in width with the spacer, thus having a pitch smaller than the lithography resolution limit. This technique is referred to as SADP (Self-Aligned Double Patterning).
An LELE (Litho-Etch-Litho-Etch) technique is known as a method of forming active areas of island patterns surrounded by isolation regions formed by an STI (Shallow Trench Isolation) method. According to the LELE method, a desired pattern is formed by combining a first pattern created by the first round of lithography and etching with a second pattern created by the second round of lithography and etching. The above SADP technique is adopted to form the first pattern, whose pitch is thus reduced to half by the SADP technique, and a part of the first pattern is cut by the second pattern. Through this process, fine island patterns having a pitch smaller than the resolution limit can be formed.
Japanese Patent Application Laid Open No. 2008-103718, Japanese Patent Application Laid Open No. 2011-233878.